1. Field of the Invention
The present invention relates to an image processing apparatus and method and to an image forming apparatus and method, and more particularly, to half-toning technology used in an image forming apparatus of a dot recording system, which is typified by an inkjet printer.
2. Description of the Related Art
An image forming apparatus has been known which forms an image on a recording medium by means of dots, such as an inkjet printer, thermal printer, or LED printer. In an image forming apparatus of this kind, an image is formed by means of a plurality of dots each of which is composed of droplets of ink liquid, toner particles, or the like, and therefore, essentially, an image is reproduced based on whether a dot is deposited or not deposited at each of locations on a recording medium, such as white paper. Since the number of inks used is limited, then a half-toning method has been used to represent continuous tonal graduation by means of a limited number of inks, or the like.
As one method for converting multiple-value image data which has a multiple-value graduated tone value for each pixel, into binary image data, Japanese Patent Application Publication No. 2004-142196 discloses a method which combines a density pattern method with an error diffusion method. In order to prevent decline in density due to defective nozzles, Japanese Patent Application Publication No. 2004-142196 proposes technology which restricts the quantization number used in the error diffusion method in order to avoid the use of nozzles suffering ejection failure.
Japanese Patent Application Publication No. 2001-054956 discloses a method in which, in order to prevent cyclical (spatially-periodic) non-uniformities which are a distinctive feature of a density pattern method, a dot pattern table which is expanded two-dimensionally is used to select the dot pattern to be adopted for each graduated tone value, on the basis of the positional information relating to the pixel.
Japanese Patent Application Publication No. 2004-058282 discloses technology whereby, in order to correct image defects, such as banding caused by variation in the volume or the ejection direction of a ink droplet ejected from one of the respective nozzles, a shading correction is applied by multiplying a correctional coefficient with respect to the processed nozzle and the surrounding nozzles thereof.
In the method disclosed in Japanese Patent Application Publication No. 2004-142196, it is difficult to correct for banding caused by recording errors other than ejection failure, in other words, to correct for banding caused by variation in the ejected volume of the ink droplet or the ejection direction. Furthermore, the technology described in Japanese Patent Application Publication No. 2004-142196 does not include the concept of using the nozzles surrounding a nozzle suffering ejection failure in order to perform correction, and the corrective performance of this technology is low.
Although the technologies described in Japanese Patent Application Publication No. 2001-054956, and Japanese Patent Application Publication No. 2004-058282 propose techniques for remedying the drawbacks of Japanese Patent Application Publication No. 2004-142196, they involve problems of the following kinds, when considering their application to a combination of an error diffusion method and a density pattern method. With regard to the terms used in the following description, when recording is carried out by moving the recording head and the recording paper (recording medium) relative to each other, the direction of this relative movement is called the “sub-scanning direction” and the direction perpendicular to this sub-scanning direction is called the “main scanning direction”.
Using the techniques described in Japanese Patent Application Publication No. 2001-054956 and Japanese Patent Application Publication No. 2004-058282, when a dot pattern table which corrects the respective nozzle characteristics and which has the number of dot pixels corresponding to the number of recording elements in the main scanning direction, and a prescribed number of dots (a sufficiently large number of dots) in the sub-scanning direction, is created for each graduated tone value, and when an image is processed by selecting a dot pattern on the basis of the graduated tone value of the corresponding input pixel and the positional relationship of the pixel (the position of the pixel in the sub-scanning direction with respect to the nozzle to be used), then a problem arises in that the storage capacity required to store the dot pattern tables becomes extremely large.
Furthermore, as for dots in the end portions of a dot pattern, the effects of dots belonging to a neighboring pixel block (dot pattern), are taken into consideration, and therefore, although there is no problem if the neighboring block and the processed block have the same graduated tone value, the corrective performance will decline in the case of dot pixels belonging to the boundaries of the block, if the neighboring blocks have graduated tone values different from the processed block. Since dot pixels (namely, pixels in the boundary sections of blocks) suffering this decline in corrective performance are arranged consecutively in the sub-scanning direction and are also aligned in a regular fashion in the main scanning direction, then there is a problem in that cyclical (spatially-periodic) non-uniformities caused by these dot pixels suffering a decline in corrective performance are liable to be visible.